Announcements for the Department of Physics at UMBC

July 2009 Archives

July 20, 2009

MS Defense - Shelly Watts

Shelly successfully defended her masters thesis on July 20, 2009.

TITLE:
A study of Surface Plasmon-Coupled Emission from Rhodamine 6G using picosecond pulses

ABSTRACT:
Fluorescence measurements are used in life sciences to provide important information
about biomolecules (fluorophores) such as structure, mobility, and conformational
changes by detecting the target molecules on surfaces. Currently, fluorescence
measurements are performed using free-space (FS) detection, which are mostly isotropic,
resulting in detection of approximately 1% of the total emission. The emission process
may be limited by the background fluorescence due to its isotropic nature and,
photochemical destruction of the fluorophores.

Surface Plasmon-Coupled Emission (SPCE) is a fluorescence technique that has been
recently introduced that increases the fluorescence yield. SPCE is based on the
interaction of excited-state fluorophores with a nearby metal surface. The fluorophores
above the metal surface can couple with the plasmon resonances in the metal, resulting in
directional and wavelength-resolved emission. The coupled emission is characterized by
a dependence of the emission wavelength on the emission angle. In addition, the
emission is horizontally (p) polarized. An advantage of the SPCE over FS signal is the
reduction of the background fluorescence signal, since only fluorophores close to the
metal surface will couple to the surface plasmons.

Picosecond pulses were used to study the SPCE properties of Rhodamine 6G fluorophore
on a thin silver film. It is expected that using pulsed laser sources can greatly enhance
the SPCE signal over the FS signal. The SPCE signal is 3 times higher that the isotropic
FS signal. Thus, SPCE technique under pulsed excitation promises to be an effective tool
for fluorescence measurements in investigating the optical properties of biomolecules.

July 29, 2009

MS Defense - Ross Dixon

Ross successfully defended his masters thesis on July 29, 2009.

TITLE:
Forcing Mechanisms For Heavy Precipitation in the Extratropical Transition of Atlantic Hurricanes

ABSTRACT:
Freshwater flooding is the number one inland killer associated with hurricanes that make landfall in the Mid-Atlantic region. Although great improvements in hurricane track forecasting have been made over the past decade, forecasting hurricane intensity change and rainfall has remained problematic. This challenge becomes even more difficult after the storm makes landfall. Over land, storms typically weaken; however, strong nonlinear interactions with mid-latitude systems or forcing from terrain can reintensify the storm or trigger extreme precipitation events. The goal of the work presented here is to better understand the physical processes that lead to heavy precipitation and storm reintensification during the extratropical transition of hurricanes in the Mid-Atlantic region. We use the North American Regional Reanalysis to analyze in detail two landfalling storms: Hurricane Gaston (2004) and Hurricane Ernesto (2006). Both storms presented forecast challenges and both resulted in heavy precipitation, although through different mechanisms. Gaston was shown to create its own baroclinic zone, which led to heavy rainfall and latent heat release which allowed the storm to briefly rejuvenate over land. Ernesto interacted strongly with an upper level trough and jet, which created a secondary circulation that fueled the storm with moisture from the Atlantic. A potential vorticity analysis shows evidence for a case of stolen identity and possible stratosphere-troposphere exchange (STE). Diabatic forcing in the mesoscale proved to be most important in the transition of Gaston, whereas synoptic scale interactions were crucial to the evolution of Ernesto, which also occluded very quickly. The various spatial scales and rapid transitions of both these storms provide insight into the forecasting challenges during these transition events.